Oxidation resistant carbon and graphite bodies



Neal J. Johnson and John I). Niclrerson, Niagara Falls,

1N. Y., assignors to Union Carbide Corporation, a corporation of New York No Drawing. Application March 27, 1956 Serial No. 574,084

4 Claims. (Cl. 117-421) This invention has reference to a method of using phosphorus-containing organic compounds to protect baked or finished graphitized carbon bodies against oxidation.

Phosphoric acid and inorganic phosphates have been suggested and tried to protect baked or baked and graphitized carbon or graphite bodies against oxidation. Their use, however, requires that the impregnated body be heated to a sufficient temperature so that the phosphorus compounds can react to neutralize the oxidation catalytic agents normally present in the carbon or graphite body, and also to remove any solvent or diluent used to carry the phosphorus compounds. The heating temperature also must be suflicient to decompose the phosphate salt either prior to service, or during service. This technique is objectionable for two reasons. For one, heating prior to use presents an additional expensive step. For another, heating in service usually results in uncontrolled heating rates.

It is thus the primary object of this invention to overcome the foregoing and related disadvantages of prior art methods, by providing carbon and graphite bodies having improved resistance to attack by molten metals, and to oxidation by fluids and gases which oxidize carbon bodies.

A further object of the invention is to provide a method for protecting machined graphite bodies against oxidation.

The attainment of the above objects is based upon the discovery that phosphorus-containing organic compounds which are soluble or miscible with tars, pitches or resins commonly employed as impregnants to increase the density, oxidation-resistance and strength of carbon and graphite articles, can further increase oxidation resistance of graphitic and carbon bodies; and, in particular cases, render the original tar or pitch more fluid at a given temperature. It has been discovered also that these organic phosphorus-containing materials inter-act in close association with the ash components of the pitch, tar ad carbon particles. It further has been discovered that addition of the above-mentioned compounds improves the properties of the original carbon or graphite body. Thus a previously machined graphite or carbon article treated by the method of the invention thus can be strengthened, densified, as well as improved in its abrasion and oxidation-resistant properties in a single operation.

Phosphorus-containing organic compounds which are soluble in, or miscible with, molten pitch are suitable in the practice of this invention. Tri-cresyl phosphate, isooctyl, methyl acid, tri-butyl and ethyl acid phosphate perform admirably. The amount of these compounds that is used depends upon the desired end results, and the degree of protection necessary.

To provide an understanding of the invention, and for the sake of conciseness, its practice will be illustrated mainly by reference to the performance of tri-cresyl phosphate and of methyl acid phosphate.

In one example of the invention, carbon specimens two inches in diameter and four inches long, in the one case were impregnated with coal tar pitch alone for control purposes, while others were treated with pitch containing 5 percent and 20 percent by weight respectively of tri-cresyl phosphate. The specimens gained in weight by approximately 16.5 percent after this treatment. All these were then re-baked to 900 C. to carbonize the pitch, and were then tested for their rate of oxidation in air at 630 C. Table I relates the percentage weight losses after various lengths of time at 630 C.

TABLE I Oxidation of graphite at 630 C. in air The pitches used in the present invention can be any of the conventional types, in their various melting point ranges, Naturally conventional plasticizers may be used with the selected pitches.

Oxidation tests Were also performed on 1 x 1 x /2 inch graphite specimens treated with coal tar pitch alone for control purposes, and with others treated with pitch containing 8 percent by weight of tri-cresyl or methyl acid phosphate. These samples were then re-baked to 800 C. to carbonize the pitch in the graphite pores. Oxidation results at 660 C. in air appear in Table II following.

TABLE II Oxidation of graphite at 660 C. in air Percent Weight Loss Atter- Graphite Specimens 1% 492 9% Hours Hours Hours Control (Treated with coal tar pitch It is important in the practice of this invention to limit the temperature at which the articles are baked, after impregnation, to a degree not sufiicient to volatilize the phosphorus-containing radical from the final graphite or carbon article. This baking temperature limit is approximately 1200 C.

In addition to the beneficial results indicated above, the practice of this invention in some instances facilitates the impregnation of graphitic carbon bodies. This is the case because pitch to which plasticizing phosphates have been added is more fluid at a given temperature, and more amenable to handling operations.

The modified pitch of the invention also produces more coke from the pitch, for comparable quantities of pitch, in the carbon pores after baking than is the case with conventional pitch. Presence of the additive in some instances contributes to an increased density, and generally the finished product has a strength superior to conventionally prepared products.

What is claimed is:

1. A method for rendering carbon and graphite articles oxidation resistant, which method consists in impregnating said carbon and graphite articles with pitch containing a phosphorus-containing organic compound miscible therewith, and then baking the graphite or carbon body to a temperature below 1200 C.

2. A method for rendering carbon and graphite bodies oxidation resistant, which method comprises impregnating said bodies with a mixture consisting of pitch and between 5 percent and 20 percent by weight thereof of at least one pitch-miscible organic phosphorus-containing compound selected from the group consisting of tri-cresyl phosphate, iso-octyl phosphate, methyl acid phosphate, tri-butyl phosphate and octyl di-phenyl phosphate, and then baking the graphitic body.

3. In the method of rendering graphitic carbon bodies oxidation resistant by impregnating the same with pitchlike materials, and then carbonizing the thus treated body, the improvement which consists in using pitch having between 5 percent and 20 percent by weight thereof of a phosphorus-containing organic compound miscible therewith.

4. In the method of rendering graphitic carbon bodies References Cited in the file of this patent UNITED STATES PATENTS 1,617,195 Matter Jan. 4, 1916 1,462,003 Bleecker July 17, 1923 2,636,836 Whigham et a1 Apr. 28, 1953 2,648,644 McMillan et a1 Aug. 11, 1953 2,685,542 Woodburn et al. Aug. 3, 1954 2,693,425 Hardman Nov. 2, 1954 OTHER REFERENCES Asphalts and Allied Substances (Abraham), published by D. Van Nostrand Company, Incorporated (New York), 1945, fifth edition, volume 1 (pages 527 and 528). 

1. A METHOD FOR RENDERING CARBON AND GRAPHITE ARTICLES OXIDATION RESISTANT, WHICH METHOD CONSISTS IN IMPREGNATING SAID CARBON AND GRAPHITE ARTICLES WITH PITCH CONTAINING A PHOSPHORUS-CONTAINING ORGANIC COMPOUND MISCIBLE THEREWITH, AND THEN BAKING THE GRAPHITE OR CARBON BODY TO A TEMPERATURE BELOW 1200*C. 